BASIC GAS LAWS - 2012 NGA GAS OPERATIONS SCHOOL
There are a number of very important reasons why everyone connected to the natural gas industry should understand the Basic Gas Laws. (1) Designing, building, and maintaining safe gas distribution and transmission systems can only be done by understanding and applying the laws of physics governing the behavior of natural gas. (2) Effective job performance in the natural gas industry requires an understanding of the technical language we encounter every day, a substantial portion of which is directly and indirectly related to the behavior of natural gas. (3) Safety is perhaps the most important driving force in our industry and many of the safety issues employees face relate to understanding how gases behave. (4) Many of the service issues arising from interaction with gas customers involve questions of measurement accuracy, pressure, and terminology that are not at all understood by end users but are related to the application of Basic Gas Laws.
This class is not intended to cover the subject matter in depth but instead will focus on
(3) Basic Gas Laws that everyone should conceptually understand.
Of the (3) Basic Gas Laws discussed in this presentation, Boyle's Law describing the effect of pressure on gas is the most commonly applied and far-reaching in its impact in our industry.
Therefore it will get the most attention in th i s presentation.
For those who are not engineers or mathematically inclined, a very simple example of Boyle's Law at work in the real world is the use of scuba tanks by divers that need to work (and breathe) underwater.
A scuba tank's internal volume capacity is relatively small (~ypically less than % cubic feet) so how is it that a diver can stay underwater for so long breathing from this tank? The answer is that the tank is pressurized to between 3000 and 4000 psig. While the tank volume remains constant, the compressibility of air means that we can "cram" a lot of air into the tank by applying lots of pressure. Boyle's Law is a way to measure the change in volume that occurs with a change in pressure.
"At a constant temperature.
the volume of an ideal gas decreases when an increase in pressure is applied."
(Wikipedia)
Mathematically, this theorem can be expressed as follows:
Pl x Vl
=
P2 x V2
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BASIC GAS LAWS - 2012 NGA GAS OPERATIONS SCHOOL
In plain language, Boyle's Law states that if we take a given volume of gas at a constant temperature and increase the pressure applied against that volume, the molecules of gas will move closer together and the gas will take up less space. In gas distribution and transmission systems, the physical space within the piping systems is fixed and constant.
Therefore, when we increase pressure on the system, we are increasing the density of gas traveling through the piping network.
One of the terms regularly used in the gas industry is "pressure factor".
This term is expressed numerically and represents the relative increase in
volume that a system can deliver at a given higher pressure. Before citing an example of how pressure factors are calculated and what they mean, it is important to note that there are several kinds of pressure to understand.
The first is
pressure (atm.), which exists all around us.
This is the pressure the weatherman talks about on the news and it is affected by both local weather conditions and altitude.
For simplicity purposes, most companies use an
annual atmospheric pressure value calculated for their service area; here in New England
14.73psia is often used for rough calculation purposes.
However, by industry convention most manufacturers use an atmospheric pressure value of 14.4psia when calculating flow and volume capacities of their products used under varying line pressure scenarios.
A second type of pressure is
pressure, which is what most people in the industry think ofwhen we discuss the pressure within a gas piping system. This "line" pressure is usually expressed in pounds per square inch gauge (psig). However, when talking about old, lowpressure gas systems (usually cast iron), you will also hear the term "inches of water column".
One pound of pressure is approximately equal to 28 inches (27 .
72 to be precise) of water column (w.c.) pressure. Therefore, when talking about low pressure delivery systems you may hear people use such terms as .25PSIG or 7"w.c. to describe their operating conditions.
A third type of pressure is
pressure, expressed as pounds per square inch absolute (psia). This pressure is the sum of both the atmospheric pressure and the gauge pressure in the pipeline.
When discussing Boyle's Law and using it to calculate pressure factors, it is important to remember that this law uses
units of pressure, not gauge units, as its reference point.
A fourth type of pressure is called
and it is an arbitrary pressure value used to establish tariff rates for custody transfer measurement purposes (as a general rule of thumb, the most frequently used base pressure value in the natural gas industry is 14.73). The best way to think about base pressure is that it is a standardized set of conditions used to measure gas volumes that are being exchanged.
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BASIC GAS LAWS - 2012 NGA GAS OPERATIONS SCHOOL
Getting back to our original discussion point, what exactly is a pressure factor, how is it calculated, and how is it used? A pressure factor is a number that reflects the increase in standardized volume capacity that occurs when we are able to raise system pressures above
"standard operating conditions {usually 7"w.c.}.
Boyle's Law tells us that the amount of gas we can push through a pipeline, adjusted to standardized conditions, is directly proportional to the increase in pressure applied to the pipeline.
The pressure factors calculated from a change in operating conditions are regularly used to quantify potential system capacity, the size of piping needed for mains and services and to assess the proper sizing and selection of pressure regulators and meters to ensure the equipment can safely operate under the new conditions.
Following is the formula used to calculate pressure factors, along with a brief table of some of the most common pressure factors:
Atmospheric pressure (14.4) + line pressure (2)
Base pressure (14.73)
pressu re factor (1.113)
PSIG
1
2
5
10
20
30
60
PF
1.045
1.113
1.317
1.656
2.335
3.014
5.051
Pressure is absolutely essential to the operation of gas distribution and transmission systems. Without it, gas does not flow.
But higher pressures create problems as well, including higher risks of gas leaks and/or explosions.
You may have heard the term IIMAOP" {Maximum
Allowable Operating Pressure} used in conversations about gas systems. This is a pressure rating designation that stipulates the maximum pressure at which a system can be operated, based on the current design of the system. There are many factors that influence a system
MAOP, among them the pressure rating and capability of the ex i sting piping and other equipment {regulators, gas meters} that will be operating under those pressures. Damage and/or repairs to system piping as well as equipment replacements can also impact MAOP. The
MAOP is really a function of the lowest pressure-rated components in the system, which means that a lot oftime, man-hours, and money go into designing and maintaining accurate MAOP ratings.
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BASIC GAS LAWS - 2012 NGA GAS OPERATIONS SCHOOL
If higher pressures pose more risks, why don't LDCs (local gas distribution companies) and pipeline operators just reduce their MAOPs? There are (2) primary reasons. First, at lower pressures, the volume capacity of pipelines is also much lower, which means that larger diameter piping must be used to transport the gas. For example, in many old distribution systems, you will find very large diameter cast iron pipe, typically as large as 42", in use and capacities are still constrained.
Second, the cost to install large diameter pipelines is much greater than the cost of smaller diameter piping. Increasing system capacities by increasing system pressures overall is the most cost effective solution to transporting larger volumes of gas and to maintaining desired de l ivery pressures.
CHARLE'S LA W TEMPERATURE
The impact on the natural gas industry of changes in gas temperature is not as significant as that of pressure changes. However, it is important to understand that temperature is a very significant factor in overall measurement accuracy and in applications where large reductions in gas pressure (pressure cuts) occur. Unlike the effects of Boyle's Law, where increases in pressure correspond to decreases in gas volume, the inverse occurs due to increases in gas temperature.
of
of
on
(Wikipedia)
Mathematically, this theorem is expressed as:
VlxT2 = V2xTl
In plain language, Charles' Law states that if we take a given volume of gas at a constant pressure and increase the temperature applied against that volume, the molecules of gas will move further apart and the gas will take up more space. In gas distribution and transmission systems, the physical space within the piping systems is fixed and constant.
Therefore, when we increase temperature in the system, we are decreasing the density of gas traveling through the piping network.
The scientific work behind Charles' Law is based on the Kelvin temperature scale where temperatures are expressed in degrees Celsius (0C) rather than degrees Fahrenheit (OF)with which we are all familiar.
For our purposes in the U.S. gas industry, we typically use degrees
Rankin (OR)instead of Celsius because conversion is much easier to remember.
If we add 460 to a temperature in OF,we convert it to OR,for example, 32°F
=
492°R or 60°F
=
5200R.
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BASIC GAS LAWS - 2012 NGA GAS OPERATIONS SCHOOL
Rather than getting hung up on calculations to determine the impact oftemperature changes on gas volumes, there is a simple rule of thumb to remember:
For every 5°F change in gas temperature.
the volume of gas changes by 1%.
The other situation where temperature impacts gas system design involves locations where large pressure cuts occur, e.g. city gate stations and farm taps. Without going into too much detail, the general rule of thumb to remember is:
For every
psig reduction in gas pressure. there occurs a temperature drop of rF.
The reason this is important is that these temperature drops, in combination with winter operating conditions, can often result in ice buildups on pressure regulating and metering equipment, increasing the chance that equipment operation may be impaired.
Where these conditions occur, you will often find the equipment fitted with some type of heating system or placed inside heated buildings.
(the "fudge" factor}
You may recall that both Boyle's and Charles' Laws define the behavior of "ideal" gases.
In the real world, these "ideal" gases do not exist. As a result, we have learned that the behavior of gases does vary from what those "Laws" mathematically predict.
In fact the deviation from those models is rather complex and NOT linear at all .
The amount of deviation that occurs is a function both of high pressures and high temperatures and can result in very large changes in volume.
The measurement of the effects of super compressibility has substantially improved due to the use of more sophisticated pressure and temperature sensors and the complex algorithms written in software programs.
For our purposes today, you just need to be aware that the effect of super compressibility is approximately a 1% increase in volume at 60 psig and can increase to over 10% at transmission operating conditions.
vs.
One of the most widely used and frequently misunderstood concepts in the gas industry is the term "volume", typically expressed in terms of a rate of flow in cubic feet per hour (cfh).
By now, you should have recognized that natural gas volumes are different from volumes of liquids or solids because they are more significantly impacted by changes in pressure and temperature.
In order to design and build gas piping systems and exchange gas among different parties, it is essential that we define volume in terms that are consistent and
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BASIC GAS LAWS - 2012 NGA GAS OPERATIONS SCHOOL understood by all parties.
For this purpose, the gas industry uses what is called a
"standardized cubic foot" of volume (scO. It is defined as the amount of gas found within a cubic foot (cO volume container when the temperature of the gas is fixed at 60°F and the base pressure conditions are adjusted to 14.73
psia.
It is important to remember that the natural gas network
in cubic feet units of volume but system engineering design and custody transfer require mathematical conversion ofthose units to standardized cubic feet.
COMMON GAS INDUSTRY DEFINITIONS
the volume of a container that is l' high, l' wide, and l' deep (cf)
the volume of a container that is l' high, l' wide, and l' deep when held at a constant temperature of GO°F and base pressure conditions of 14.73
psia (scf).
British thermal unit, represents the amount of heat required to raise the temperature of 1 pound of water from 39°F to 40°F.
1 scf of natural gas contains
N
1,028 Btus
the unit of volume reco r ded on gas meter indexes, representing hundreds of cubic feet
a function of both the energy value and the volume of natural gas, it is equal to 100,000 Btus. (NOTE: based on average heating values, a ccf is approx i mately equal to 1 therm.)
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